Sahih Muslim Book 4, Hadith Number 796.

Chapter : The placing of the right hand over the left hand after the first Takbir in prayer (Takbir-i-Tahrima) below the chest and above the navel and then placing them opposite the shoulders in prostration.

Abdullah b. Mas’ud reported: We were sitting with the Apostle (may peace be upon him) in prayer, and the rest of the Hadith is the same as narrated by Mansur He (also said): After (reciting tashahud) he may choose any prayer.